Magnetic coating compositions



United States Patent 3,419,420 MAGNETIC COATING COMPOSITIONS FrederickA. Stahly and Wesley Dale Humphriss, Rochester, N.Y., assignors toEastman Kodak Company, Rochester, N.Y., a corporation of New Jersey NoDrawing. Filed Nov. 2, 1964, Ser. No. 408,336 13 Claims. (Cl. 117138.8)

ABSTRACT OF THE DISCLOSURE A magnetic recording tape having a coating ofmagnetic oxide particles bound by a poly (vinyl butyral) resin adsorbedto the oxide particles in a ratio of 10 to 70 or more milligrams ofbinder per gram of oxide. The resin should contain 5 to 17% poly (vinylalcohol). Advantageously, the ratio of magnetic oxide particles tobinder is between 2 and to 1. The coating is prepared by forming adispersion comprising a thixotropic solution of the resin, a nonpolarsolvent, and the magnetic oxide particles, subjecting the dispersion toshear forces to decrease its viscosity, coating the resulting thinneddispersion onto a tape base, and allowing the thixotropic dispersion toagain become viscous while the solvent evaporates.

This invention relates to coating compositions and magnetic recordingtapes utilizing such coating compositions. More particularly, thisinvention pertains to coating compositions wherein the binder is presentin unusually small relative amounts and is adsorbed to the magneticoxide, and to magnetic recording tapes using such composition to producea very smooth surface and homogeneous finegrained internal structure.

Magnetic recording tapes are generally made by dispersing magneticallyactive particles throughout a neutral binder to form a mixture and thencoating the mixture upon a pliable film or web carrier. The ratio ofmagnetic particles to binder is usually in the range of 221-411 on aweight basis. As might be expected, certain desirable properties of therecording tape are enhanced when the relative amount of magnetic oxideparticles is increased and other desirable properties are obtained whenthe relative amount of binder is increased.

For instance, increasing the amount of magnetic oxide in the compositionnormally increases the frequency response and quality of playback signalobtainable without distortion. But this basic and vital improvement isusually obtained only at the expense of decreased wear resistance of thetape.

Increased wear resistance is generally obtained by increasing therelative amount of binder and usually results in a correspondingdecrease in signal strength and quality from the resulting tape.

Thus it can be seen that the magnetic oxide-binder ratio is usuallydetermined on a compromise basis with none of the funadamentalproperties of the tape being optimized though the ratio can be shiftedto improve certain properties vital in specific uses at the expense ofless important properties for that use.

There is yet another common difliculty encountered in producing magneticrecording tape. That is the propensity of the magnetic oxide to formaggregations of particles after coating before the binder sets. It willbe recognized that the magnetic particles often have a mutual attractionand, as a practical matter, some time must be allowed after coating forthe binder to set so that it does not irreversibly set before or duringcoating of the carrier. Clurnps of magnetic oxide often form before thebinder sets. Lack of homogeneous dispersion of the magnetic oxidescauses decidedly inferior performance of the tape 3,419,420 PatentedDec. 31, 1968 since areas of the tape will contain no recording mediumwhile other areas will contain aggregates of particles which cannot befully and effectively utilized. Uneven dispersion of the magnetic oxideparticles adversely affects noise ratio and detracts from the quality ofthe tape.

It is an object of the present invention to provide a coatingcomposition having a very high magnetic oxide-tobinder ratio wherein thephysical qualities of the tape are enhanced as well as theelectroaco-ustical qualities.

A further object of the invention is to provide a magnetic tapedisplaying superior wear resistance.

A still further object of the invention is to provide a magnetic tapehaving greater strength than conventional tapes.

Another object of the invention is to provide a composition wherein themagnetic oxide particles can be rendered immobile relative to oneanother either in the coating solution or immediately upon coatingindependently of the setting of the binder so that the immobility can bereversed upon performing a simple physical process step.

These objects are accomplished by the present invention by forming acoating composition having a magnetic oxide-to-binder ratio of between2: l10:1 of a magnetic iron oxide and a specific binder which willadsorb from 10-70 or more milligrams of binder polymer per gram ofmagnetic oxide. The preferred adsorption level is from 25 to 45milligrams of binder per gram of magnetic oxide.

To accomplish the abnormally high specified levels of adsorption, it isnecessary that specific poly (vinyl butyral) compositions within acertain range be employed as the binder. Further it is imperative thatspecific types of solvents be avoided in preparing the composition. Ofcourse the solvent will not remain in the composition after coating onthe carrier, but the high levels of adsorption cannot be reached unlessspecified solvents are avoided while preparing and coating thecomposition.

The specific binder of the instant invention is a poly (vinyl butyral)which contains 5-17 percent poly (vinyl alcohol), and preferably 9-13percent poly (vinyl alcohol). It has been found that this binder adsorbsto magnetic oxide particles in large quantities. Such adsorption permitsthe use of relatively less binder without compromising the wearproperties in the magnetic tape. A somewhat similar situation appears inthe compounding of rubber wherein the addition of carbon black particlescauses the vulcanized rubber to be much tougher than a similar compoundwithout the carbon black. The roughly similar use of magnetic oxideparticles to enhance the strength of the binder in the present inventionis obviously a most significant improvement.

Only certain type of solvents may. be employed in the present inventionif the adsorption of poly (vinyl butyral) to iron oxide is not to bepoisoned. The adsorption level is roughly inversely related to thepolarity of the solvent. Polar solvents such as alcohols includingethanol are inoperative in the invention. Methyl isobutyl ketone,acetone, ethyl acetate, toluene, 1,1,1 trichloroethane, 1,1,2trichloroethane and mixtures thereof are convenient examples of the manyworkable relatively nonpolar solvents. All of the solvents and solventmixtures useful in the invention promote adsorption of poly (vinylbutyral) to magnetic oxide. As stated above, the adsorption must be atleast 10 milligrams of polymer per gram of iron oxide.

Adsorption is a natural condition between the specified poly (vinylbutyral) and magnetic oxide. However, if care is not exercised inchoosing the proper solvent, the adsorption sites on the magnetic oxidewill be occupied by polar groups of the solvent. Such poisoning of theadsorption sites renders the principle of the present inventioninoperative.

Numerous solvents do operate satisfactorily in the instant invention. Itwould be unrealistic to attempt to list the thousands of suitablesolvents and combinations thereof. Since the choice of solvent orsolvent systems often depends on different circumstances, such as thechoice of carrier or economic considerations, no single solvent orsolvent system is clearly superior for all uses. However, preferredsolvents and solvent systems are methyl isobutyl ketone, methyl isobutylketone with -20 percent acetone, methyl isobutyl ketone with 0-50percent ethyl acetate, toluene, toluene with 0-20 percent acetone,toluene with 0-50 percent ethyl acetate, 1,1,l-trichloroethane,1,l,l-trichloroethane with 050 percent 1,1,2-trichloroethane.

Workable solvents and solvent systems can be easily determined bypreparing a 1.0 percent by weight solution of the specified poly (vinylbutyral) binder in the solvent system of concern. Ten grams of magneticoxide powder are added to 50 grams of the prepared solution. The mixtureis then agitated and allowed to come to equilibrium, preferably byallowing it to stand overnight, but at least for 2 /2 hours. Afterequilibrium is reached, the solution is centrifuged for minutes tocompletely settle the iron powder. The supernatent liquid is thenevaporated, and the polymer remaining in solution thus determined. Ithas been found that the dilference between the remaining polymer andthat originally in the solution is an accurate indication of the polymeradsorbed to the magnetic oxide. If at least 10 milligrams of polymer pergram of magnetic oxide is adsorbed, the solvent is nonpolar for thepurposes of the invention. Preferred solvents are those which allowadsorption of at least 25 milligrams of poly (vinyl butyral) per gram ofmagnetic oxide. As a general rule it has been found that the so-calledstrong or good polymer solvents adversely affect the desired adsorptionwhile the so-called weak or poor polymer solvents permit highadsorption.

A variety of commercial magnetic oxides as well as other proprietarymagnetic materials can be utilized in the instant invention. A preferredmaterial is gamma Fe O These magnetic oxides are widely available anddisplay desirable adsorption characteristics unless treated, such aswith the above discussed strong solvents, to saturate or decrease thenumber of adsorption sites.

The magnetic oxide-poly (vinyl butyral) composition of the instantinvention can be deposited on many pliable carriers. Conventionalcarriers such as poly (ethylene terephthalate), cellulose acetate andother conventional :pliable polymer carriers are suitable for use withthe herein composition.

In addition to the concurrent improvement of physical andelectroacoustical properties resulting from the stronger binders, theabove discussed compositions displays yet another advantage. As a resultof the high adsorption level between the poly (vinyl butyral) binder andmagnetic oxide, the composition, when dispersed in a suitable nonpolarsolvent, forms a thixotropic solution. In such a solution, the magneticoxide particles are held immobile even while the binder is unset.However, when subjected to shear, the thixotropic solution undergoes alarge decrease in viscosity. In effect, the solution is a solid untilagitated by shear whereupon it becomes liquid in nature. The solutionsof the composition of the instant invention are subjected to shearimmediately before coating. Thus, while coating, the solution functionsas a liquid. However, when the coating is accomplished and the solutionis no longer subjected to shear, the viscosity again increases therebypreventing grouping of the magnetic oxide particles in the freshcoating. The prevention of grouping or clumping is not a result of thebinder setting, but primarily the result of the coating reverting to itssolid thixotropic state. Greatly improved homogenity results from thiscoating technique utilizing the thixotropic nature of this solution.

Thixotropic solutions are often avoided because they are more difficultto work with. However, the substantial increase in quality more thanoutweighs the minor increase in complexity of the coating operation.Apparatus such as disclosed in Lawrence C. Bartlett et al., US. Patent3,227,136, granted Jan. 4, 1966, for Extrusion Coating Apparatus iscapable of producing high quality magnetic tape with the thixotropiccomposition of the instant invention.

The invention will be more easily understood upon consideration of thefollowing examples.

EXAMPLE I A coating composition for magnetic tapes was prepared from thefollowing composition.

Percent by weight Butvar B-76 6.6 Magnetic iron oxide 33.4 Methylisobutyl ketone 51.0 Acetone 9.0 Oxide to binder ratio, 5/1.

Butvar B-76 is a commercial poly (vinyl butyral), having a hydroxylcontent expressed as percent poly (vinyl alcohol) of between 9 and 13,produced by the Shawinigan Resins Corporation. It is a preferred binderfor use in the instant invention. The above specified formula wasballmilled seven days until complete dispersion was accomplished. It wasthen coated onto a poly (ethylene terephthalate) carrier usingthixotropic techniques and allowed to harden. The thus formed magnetictape Was found to have excellent properties.

EXAMPLE II A magnetic tape such as that described in Example I wasprepared utilizing a poly (vinyl butyral) having a poly (vinyl alcohol)content of 2 percent.

EXAMPLE III A magnetic tape such as that described in Example I wasprepared utilizing a poly (vinyl butyral) having a poly (vinyl alcohol)content of more than 17 percent. The poly (vinyl butyral) in thisexample was Butvar B- which has a hydroxyl content expressed as percentpoly (vinyl alcohol) of between 18 a'nd 20.

EXAMPLE IV A magnetic tape as in Example I was prepared except that theacetone in the coating composition was replaced with methyl alcohol.

EXAMPLE V The thixotropic coating composition of Example I was used toproduce a magnetic tape using a non-shearing coating technique.

The electroacoustical characteristics of the magnetic tapes produced inthe above example are compared in the following table.

TABLE I.ELECTROACOUSTICAL RESULTS Exaan- Exam- Comple I ple II InercialTapes Bias O o 0 0%. Low frequency sensitivity, 400 c.p.s +1.0 db 0 0db. High frequency sensitivity, 15 kc./s +2.0 (1 0 0 db. Frequencyresponse +1.0 db 0 0 db. Undistorted output..." +1.0 db 0 0 db.Saturated output +1.0 (11)... 0 0 db. Low frequency signal to noiseratio, Mod- +2.0 db- 0 0 db.

ulation 0-1 kc. High frequency signal to noise ratio, +2.0 db... 0 0 db.

Modulation 1-20 kc. Zero signal-signal to noise ratio +2.0 db 0 0 db.

Noris.Examples III, IV, VNo samples to test because dispersions ofExamples 111 and IV were too poor to coat and coating of Example V wastoo poor to test.

Thus it is clear that the electroacoustical properties of the magnetictape produced according to the instant invention, i.e., Example I, areimproved over those of commonly available commercial tapes. Further, itis apparent that the hydroxyl content expressed as poly (vinyl alco- 7.A coating composition as set forth in claim 4 wherehol) in the poly(vinyl butyral) is a critical consideration. in the magnetic oxideparticles are gamma Fe O The physical characteristics of the tapeproduced in 8. A magnetic recording tape comprising a magnetic the abovesheet are described in the following table. coating compositionconsisting essentially of a binder in 5 minor proportion and magneticoxide particles in major TABLE H PHYSICAL RESULTS proportion disposed inan adherent coating upon a pliable carrier, said binder consistingessentially of pol (vinyl Examplel Example H $323,1 butyral) resincontaining 517% poly (vinyl zilcohol),

said binder being adsorbed to said oxide particles in a Cohesivestrength ratio of at least 10 milligrams of binder per gram of ExcellentGood Wear Resistance do Moisture Resistance 0 .d0 D0: magnetic xideparticles I s ii fi98353312213111::3:338:31:1111133113: 138: A magncticrecording p as set forth in claim 8 NorE.-Examples III, IV, VNo samplesto test because dispersions Wherem Sald poly (Vmyl butyral) has a p 01y(vmyl alcoof Examples III and IV were too poor to coat and coating ofExample V content of P was too p to test- 15 10. A magnetic recordingtape as set forth in claim 8 wherein said magnetic oxide particles aregamma Fe O The rating good for commercial tapes is a typical ac- 11. Amagnetic recording tape as set forth in claim 8 ceptable value for agiven property although many com- Wherein said pliable carrier is poly(ethylene terephmercial varieties are relatively poor in one or more ofthalate). these, physical roperties, 12. A magnetic recording tapecomprising a poly (vinyl Thus the physical characteristics of the tapeproduced butyral) binder having a poly (vinyl alcohol) content ofaccording to the instant invention are improved relative 9-13 percent,gamma Fe O particles dispersed throughout to both commercial tapes andtapes similar to those of said binder in a particle-to-binder ratio of10:1-2z1, and the invention, but outside of the specified criticallimits. a pliable poly (ethylene terephthalate) carrier, said binderAlso, the importance of using thixotropic coating methods and particlesbeing disposed as an adherent coating on is evident. said carrier.

As Will be evident to those skilled in the art, the above 13. A methodof preparing magnetic recording tape description and examples are merelyillustrative and not comprising forming a thixotropic dispersionconsisting intended to limit the invention. essentially of a majorproportion of magnetic oxide par- We claim: ticles, a minor proportionof binder consisting essentially 1. Amagnetic recording tape coatingcomposition comof l i l ibutyral) e in ontaining 5-17% poly prisingmajor Proportion of magnetic Oxide Particles (vinyl alcohol), and anonpolar solvent, subjecting the and a minor Proportion of a binderConsisting essentially thus formed dispersion to shear forces todecrease the of a poly (vinyl butyral) binder containing 517% poly(vinyl alcohol), said binder being adsorbed to said oxide particles in aratio of at least 10 milligrams of binder per gram of magnetic oxideparticles.

2. A coating composition as set forth in claim 1 wherein the magneticoxide particles are gamma Fe O 3. A coating composition as set forth inclaim 2 whereviscosity of such dispersion, coating the dispersion onto apliable carrier while the viscosity is decreased, and allowing thethixotropic dispersion so coated to again become viscous While thesolvent evaporates.

References Cited in the binder is adsorbed to the Fe O in the range ofUNITED STATES PATENTS from 25 to milligrams of binder per gram of Fe O2,666,719 1/ 1954 Lissant 117-132 4. A magnetic recording tape coatingcomposition com- 2,914,480 11/ 1959 Hagopian 252-62.54 prising a majorproportion of magnetic oxide particles 3,080,319 3/1963 Arrington274-41.4 dispersed in a minor proportion of a thixotropic solution 453,135,625 6/1964 Ingrassia 1l7-62.2 of a resinous binder consistingessentially of a poly (vinyl 3,240,621 3/ 1966 Flower et al. 11793.2butyral) resin containing 517% poly (vinyl alcohol), 3,243,376 3/1966Lovick et a1 252-62.53 and a nonpolar solvent. 3,216,846 11/ 1965Hendricx et al. 117-62 5. A coating composition as set forth in claim 4wherein the magnetic oxide particles-to-binder ratio is within WILLIAMMARTIN, Primary Examine!- the range 10: B. PIANALTO, Assistant Examiner.

6. A coatlng composition as set forth 1n claim 5 wherein the nonpolarsolvent comprises at least one of the class CL including methyl isobutylketone, acetone, ethyl acetate,

toluene, 1,1,1-trichloroethane, or 1,1,2-trichloroethane. 117121, 5,161; 252-6 2.54

